1. Field of the Invention
The present invention relates to an optical disk capable of recording information in the form of a row of marks having a reflectivity different from that of the surrounding and, more particularly, to an optical disk of a groove recording type.
2. Description of the Related Art
Conventionally, there have been developed optical disks, such as DVDs (Digital Versatile Disks) as optical mediums to record information with a high density. In the DVDs, write-once type DVD-Rs (Digital Versatile Disk-Recordables) and rewritable DVD-RWs (Digital Versatile Disk-ReWritables) have been classified to groove recording type optical disks in which a groove is used for a guide. These groove-recording-schemed DVDs are manufactured under specifications of a track pitch of 0.74 xcexcm, a data bit length of 0.267 xcexcm and a groove depth of nearly 25 nm. The DVD has 4.7 GB in one surface, for record of MPEG 2 image signals for nearly 2 hours.
The information recording and reproducing apparatus for such a DVD has a behavior of data recording as follows: The apparatus irradiates and focuses a recording light beam according to data on a groove track GV defined between lands LDs as shown in FIG. 1 to identify a position on the groove track GV by detecting a land prepit (not shown) from the DVD. At this time, the portion on which the recording light beam has been irradiated is heated, thereby forming a record marker portion M whose reflectance differs from the ambient reflectance on that portion of the groove track GV.
The information recording and reproducing apparatus has a pickup device which includes e.g. a 4-quadrant photodetector 1 as shown in FIG. 2. The 4-quadrant photodetector 1 has a photoelectric converting device having four light-receiving surfaces 1a to 1d split into four segments by a direction along the groove tracks GV of the DVD and a direction perpendicular to the groove tracks. The light-receiving surfaces 1a and 1d are positioned in the outer track side of the DVD while the light-receiving surfaces 1b and 1care positioned in the inner track side of the DVD.
A read light beam generator irradiates a read light beam onto the DVD that is rotated by a spindle motor, thereby forming a beam spot on the recording layer. The photoelectric converting device detects the intensity of reflected light of the information reading spot from the DVD at the four light-receiving surfaces 1a-1d and outputs reception signals Ra to Rd or electric signals corresponding to the amounts of light respectively detected by the light-receiving surfaces 1a-1d. The reception signals Ra and Rd associated with the light-receiving surfaces 1a and 1d positioned on the outer track side of the DVD are supplied to an adder 2, and the reception signals Rb and Rc associated with the light-receiving surfaces 1b and 1c positioned on the inner track side of the DVD are supplied to an adder 3. The adder 2 adds the light-receiving surfaces Ra and Rd, and the adder 3 adds the light-receiving surfaces Rb and Rc. Further, a subtracter 4 subtracts the output signal of the adder 3 from the output signal of the adder 2, and provides an output signal as a radial push-pull signal. Then, an RF signal, although not shown, may be produced by a total output electric signal corresponding to the amounts of light respectively detected by the light-receiving surfaces 1a, 1b, 1c and 1d. In addition, the push-pull signal changes in response to asymmetry with respect to the groove of the optical disk, because the light spot having the intensity distribution on the light-receiving surfaces is formed by a refracted light form the groove. The asymmetry of intensity distribution in a light spot on the light-receiving surfaces is caused from a phase difference between 0th diffraction light and xc2x11th diffraction light that appears when a focused light spot on the optical disk irradiated by the pickup is shifted from the groove.
The conventional groove recording optical disks such as DVD-R/RWs can record images in a high-vision digital broadcast quality for a little less than 30 minutes only. In order that a next-generation-optical-disk recording system achieves a practical recording time for the high-vision digital signal, there is a need to use a blue laser and an objective lens having a high numerical aperture NA in the pickup to further decrease a focused spot diameter and correspondingly increase the recording density to the optical disk. Generally, the optical disk can be increased in recording density by increasing the numerical aperture NA of the objective lens and shortening the wavelength xcex of the laser used in recording/reproducing. This is because the beam spot on an optical disk decreases in proportion to xcex/NA. Consequently, the recording/reproducing density of an optical disk can be improved by increasing NA and decreasing xcex. The available DVD at present has a light transmissive layer having a thickness of 0.6 mm between an outer surface and a reflection film (recording layer) under a specification of xcex=650 nm of the laser light beam and NA=0.6 of the objective lens in the optical disk system.
If the data bit length of DVD-R/RW is reduced together with the reduction in the optical spot diameter to an analogous form in order for increasing density of marks, then it is 0.117 xcexcm/bit. However this condition is difficult to record marks because of limitation of recording property. It has been reported that the data bit length as long as nearly 0.130 xcexcm/bit makes possible stable recording/reproducing. For example, in the case of (1, 7) RLL modulation, the conversion into a channel bit length provides 86.7 xcexcm/bit.
Because there is a limitation in increasing density of marks in a disk tangential direction, mark density is increased in a disk radial direction. Namely, there is a need to narrow the track pitch TP. When the track pitch is further narrowed, then the push-pull signal obtained by a pickup is reduced in the vicinity of xcex/16n (wherein xcex represents a wavelength of the used light beam and n a refractive index in the light transmissive layer) of a groove depth Gd of DVD-R/RW, thus making impossible to obtain a practical tracking signal. Due to this condition, it is difficult to make the track pitch narrower any further. From these facts, there occurs a limitation for increase of recording density in the conventional DVD-R/RW and analogous-formed track pitch TP.
Accordingly, for the optical disk having a reduced configuration analogously formed from the conventional DVD-R/RW in which the track pitch and data bit length thereof are converted into the reduced ones with respect to the value of xcex/NA, it is impossible to put it to practical use as a stable recordable optical disk system having a practical recording capacity.
Therefore, the present invention has been made in consideration of the foregoing points. It is an object of the present invention to provide a next-generation optical disk capable of recording data with a high density in comparison with the conventional DVD by using a higher numerical-apertured optical system and a shorter-wavelength of the light beam for reproduction.
An optical disk of the present invention has a recording layer recording information as a row of marks and non-marks alternately appearing in a groove formed at a track pitch of 0.280 xcexcm or greater and a light transmissive layer formed over the recording layer, to reproduce the information due to a light beam having a wavelength in a range of 375 nm to 415 nm and illuminated by an objective lens having a numerical aperture NA in a range of 0.80 to 0.90 through the light transmissive layer to the recording layer, the optical disk characterized in that: the groove has a depth in a range of 18 nm to 32 nm.
According to one aspect of the present invention of the optical disk, said groove has a width in a range of 80 nm to 240 nm.